Explicitly inserting NULL values into NOT NULL DEFAULT 0 columns

If I insert a NULL value explicitly into a column declared to be NOT NULL
DEFAULT 0 in postgreSQL 8.4 the column ends up with the default value. If I
do the same in postgreSQL 9.0 I get an error about how I am inserting a
null value into a NOT NULL column.

i.e.: insert into table1 (column1, column2) values (0, NULL); where column2
is of type integer with attributes NOT NULL DEFAULT 0

In both cases if I just don't mention the column with these attributes the
value stored is the default value.

i.e.: insert into table1(column1) values (0); where column2 is of type
integer with attributes NOT NULL DEFAULT 0

I looked through all the release notes between the versions in question and
can find nothing mentioning this change. When did this change occur, and
can I choose to keep the behavior as it was in postgreSQL 8.4?

Thanks,

Tanmay <pgsql-general(at)postgresql(dot)org>